Reduced Blood Flow in Deep White Matter During Hypercapnia Revealed by Multi-Delay pCASL in Healthy Young Adults

Abstract

The white matter (WM) cerebrovascular response remains poorly understood compared with grey matter (GM), partly due to technical challenges in perfusion quantification. Previous studies of cerebrovascular reactivity (CVR) have mostly been performed using BOLD MRI, and it is unclear to what extent they reflect changes in cerebral blood flow (CBF). In this work, we used multi-delay pseudo-continuous arterial spin labeling (pCASL) to quantify hypercapnia-induced CBF changes ({Delta}CBF) while accounting for regional variability in arterial transit time. Twenty-five healthy young adults underwent MRI during normocapnia and hypercapnia (inhalation of a 4% CO2 gas mixture). Hypercapnia induced robust positive {Delta}CBF in cortical GM (26.7 {+/-} 13.5%), superficial WM (17.2 {+/-} 12.6%), periventricular regions (13.6 {+/-} 10.6%), and subcortical GM (25.7 {+/-} 14.1%) (all p < 0.0001). In contrast, deep WM exhibited a near-zero group-mean CBF response (1.0 {+/-} 8.9%, p = 0.57), with 10 of 25 participants demonstrating negative {Delta}CBF. Negative CBF responses were consistently localized to the corona radiata, centrum semiovale, and optic radiation. Quality-control analyses showed that deep-WM {Delta}CBF estimates are robust, supporting the reliability of these findings. Moreover, across tissue compartments, higher baseline CBF was associated with reduced hypercapnic responsiveness, and deep-WM responses were strongly coupled with cortical GM responses across individuals. These results demonstrate that hypercapnia-induced cerebrovascular responses are highly heterogeneous across tissue depths and provide evidence that negative CBF responses can occur in healthy deep WM in the presence of vasodilation elsewhere. The findings challenge the assumption of uniformly positive perfusion responses during hypercapnia and support a potential role for flow redistribution arising from regional differences in vascular resistance and reserve capacity.